When pipelines are laid under water, particularly offshore in sea water, they are usually protected against galvanic corrosion by attaching galvanic anodes made of materials such as zinc or aluminum. These anodes will preferentially corrode and thereby protect the pipeline against corrosion. Such anodes usually consist of two or more arcuate segments which encircle the pipe. Pipelines larger than about 12 inches in diameter are commonly coated with a thick layer of concrete to weight the pipeline down in the water. On these, the anodes are placed in gaps in the concrete, and do not extend above the surface of the concrete. Smaller pipelines, however, will sink of their own weight, so concrete coatings are not used. On these, the anodes have a substantially larger diameter than the pipeline. The anode protector of this invention is designed for this type of installation.
Offshore pipelines are commonly laid from a continuously moving vessel known as a lay barge. On some lay barges the joints of pipe making up the pipeline are welded together on the barge. On others, known as reel type lay barges, the joints of pipe are welded together on shore and coiled up in a coil as much as 300 feet in diameter on the lay barge. The anodes are attached to the pipeline while it is still on shore or after it is put on the lay barge. In either case, the pipeline is continuously fed off the stern of the barge as the barge moves forward in the water. The forward motion of the barge causes the pipeline to be pulled off the stern of the barge by the weight of the pipe depending from the barge. To facilitate movement of the pipeline on the deck of the steel barge, it is sometimes supported on two or more sets of rollers which are mounted on the deck. Each set of rollers may consist of two pairs of automobile wheels with rubber tires set to engage and support the pipeline. The pipeline then moves down an elongate cradle, or "stinger", which extends rearwardly and downwardly from the lay barge toward the bed of the body of water. In some cases, the weight of pipe depending from the barge is great enough to pull the pipeline off the barge too fast. In such cases a tensioning device is used to restrain the pipeline movement. Such tensioning devices may consist of spring-loaded tracks engaging the pipe, or sets of automobile tires, in either case with brakes applied as necessary to hold the pipe back.
It will be appreciated that due to wind, wave and current action there is often some lateral and vertical movement of the barge which makes it impossible to draw the pipeline off the barge in a smooth straight line at all times. The erratic motions produced under such circumstances often causes shock blows to the galvanic anodes as they pass over the rollers and down the stinger. Such shock blows tend to damage and tear the anodes loose from the pipeline. Whenever an anode is torn loose, it is necessary to stop the barge movement, weld the anode back on, and then restart the barge, all at great expense to the pipe laying operation.
In addition, the tensioning devices cannot pass over the anodes, so it is necessary to either put the anodes on after the pipe passes through the tensioner, or use two tensioning devices. If the latter option is chosen, every time an anode is reached during the laying of the pipeline, it is necessary to attach a second tensioning device on the other side of the anode, and then release the first tensioning device until the anode passes through. This is also a time-consuming and expensive operation. However, if it is not done, the axial load of the tensioner on the anode would tear it loose from the pipe. The alternative of welding the anodes on after the pipe goes through the tensioner is equally objectionable, because it slows down the laying of the pipe.